Faecal worm egg counts (WECs) are used to assess the roundworm burden in livestock. They vary according to the number and type of worms present, the age of the cattle, the length of time the cattle have been exposed to worm larvae on pasture, and various other stresses. In cattle under about 18 months there is a reasonably good correlation between average mob WECs and the number of worms in the gastrointestinal tract, but this depends largely on the type of worms present. For example, WECs correlate fairly well with worm counts for barber’s pole worms (Haemonchus placei), have a moderate correlation for small intestinal worms (Cooperia species), but a weaker correlation for small brown stomach worm (Ostertagia ostertagi). The relationship between WECs and the number of worms, therefore, must be ‘interpreted’ in light of the types of worms present and other factors to indicate whether treatment is of value.